One of the most important concerns in manufacturing a racket is how to enlarge the sweet spot.
The stiffness distribution of the racket head (stringed area) of a racket was observed by measuring the tensile strength of each section of the strings of the stringed racket. According to the measurements, the:central section of the stringed area is macroscopically much stiffer than the peripheral sections. This is thought to be a product of the vertical and horizontal length of a racket frame.
Hitting a ball oil the sweet spot provides an effective elastic rebound of the ball from the racket. However, if a ball is hit on a non-sweet spot area (off-spot area), vibrations and shocks are generated by the hit and a forceful ball cannot be returned.
Since increasing the area of a racket head is effective, the racket head of popular tennis rackets was increased from typically 90 square inches to the current 110 square inches. A tennis racket with a head area as large as 125 square inches is widely used by beginners and the aged. As the area of the racket head is enlarged, the sweet spot area also increases, so that difficult, off-center balls can be easily returned. However, the head area may become too large to play with good control and quick reaction. Thus, there is a limit to making an effective racket simply by enlarging the area of the head.
A tapered string and a stringed racket with such strings, whose central stringed area has a larger string diameter than the peripheral area, are disclosed in Published Unexamined Japanese Utility Models Sho No. 52-16848 and Sho No. 52-16849 by the inventors of this invention.
Professional tennis players who usually hit the ball on the sweet spot have been requesting a sweet spot with a more effective elastic rebound. However, in conventional technologies, it is too costly to manufacture a string which can form such a sweet spot, thus the string is not practical to manufacture.
In order to solve the problems of conventional strings and stringed rackets with such strings, this invention provides a string which can provide a large sweet spot, and a stringed racket with such strings.
Another object of the invention is to provide a string which can improve the solidity of a ball rebounded from the sweet spot of a racket strung with the strings while keeping the solidity of a ball rebounded from a non-sweet spot area to the conventional level, and to provide a stringed racket with such strings.
A third object of the invention is to provide an inexpensive and attractive string which can form a pattern on the stringed area of a stringed racket, and to provide a stringed racket with such strings.